1. Field
Example embodiments relate generally to thermal management in a battery pack of a cordless power tool system having a power tool and charger.
2. Description of Related Art
Rechargeable batteries are used in numerous cordless devices such as cordless products in the home, power tools, laptops, etc. The rechargeable batteries are typically comprised of one or more nickel-cadmium, nickel metal-hydride or Lithium-ion battery cells packaged in a battery pack. In power tool environments, these battery packs include multiple cells and are removably attachable to a power tool and a battery charger.
Battery packs for cordless power tools often are exposed to harsh conditions (e.g., thrown to the ground, dropped, etc.). Also, on a work site the battery packs may be left in the rain, set in puddle of water or even exposed to water while operating the cordless tool. For example, a reciprocating saw may be used to cut overhead pipes which may contain water. If water enters the battery pack, the electronic components inside will not function properly.
Further, as current is drawn off the batteries, i.e., discharged from the cells, to power the tool motor, heat is generated within the battery pack. Also, heat is generated within the pack as charge current flows in the cells from an attached charger. The heat created during discharge and charge operations leads to increased interior housing and cell temperatures which may have a severe effect on the life expectancy and performance of the batteries. In order for batteries to properly charge, the batteries should be below a desired threshold temperature; any differential temperature between the cells in the battery pack should be minimized. If the batteries become too hot during tool use, battery life may be cut short. Also, if a battery is below a certain threshold temperature, the battery will be too cold to charge and should be warmed before charging.
Further, the cells in cordless power tools typically have metal electrodes and an electrolytic liquid (e.g., sulfuric acid). During battery operation, the electrolysis of water may produce hydrogen and oxygen gases, which may accumulate within the battery pack. Accumulation of these gases may create a highly volatile situation, in which a spark or flame could ignite the gases to cause an explosion that can cause serious damage to the tool and/or cause injury to the tool user.
Typically, an approach to removing the gases in a battery pack has been to vent the gases through a vent hole or cap provided on the housing of the battery pack. However, in a case of a cordless Li-ion battery power tool system, the cells could vent gases that are could be at extremely high temperatures, (up to 800° C.). Gases exit the battery pack or tool at such elevated temperatures pose a danger to the tool user.